Electronic solid state, i.e., static trip units are being implemented in increasing numbers in industrial circuit breakers in place of the traditional thermal-magnetic and dual magnetic trip units to initiate circuit interruption automatically in response to an overcurrent condition. The increasing popularity of static trip circuit breakers is largely due to their inherent versatility. That is, the breaker static trip unit is readily susceptible to convenient trip setting adjustability such that a highly repeatable trip-time curve can be rather precisely tailored to the load for full overcurrent protection thereof ranging from light overload to heavy short circuit proportions. Another attractive aspect of static trip units is the fact that they can be readily and economically supplemented to afford ground fault protection, as well as overcurrent protection, to load circuits.
Typically, these static trip units are self-powered in that, rather than being powered from a separate electrical source, they receive operating power from the same source feeding the protected load circuit. More specifically, current transformers, one coupled with each phase or line conductor of the protected load circuit, are utilized to develop secondary currents proportional to the load circuit phase currents for application to the static trip unit. From these secondary currents, the static trip unit extracts sufficient energy to electrically power itself while preserving the informational or signal content thereof regarding the phase current magnitudes.
With the electronic circuitry miniaturization capability afforded by now available micro-electronic technology, the physical size of static trip units can be significantly reduced to the point where they can be incorporated in ever smaller molded case circuit breaker frame sizes. In doing so, a significant factor to be considered is the power dissipation of the trip unit itself, inasmuch as there are rather stringent limitations on the permissible temperature rise above ambient occurring within the molded case when the breaker is carrying its rated current.
It is accordingly an object of the present invention to provide a self-powered, molded case circuit breaker static trip unit having extremely low power dissipation.
An additional object is to provide a static trip unit of the above character having an improved voltage regulating power supply.
Yet another object is to provide an improved static trip unit power supply of the above character which is capable of preventing erroneous static trip unit operation.
A further object is to provide an improved static trip unit power supply of the above character which operates to protect trip unit components from malfunction and/or damage in the event of high fault currents flowing in the protected load circuit.
Still another object is to provide an improved static trip unit power supply which is efficient in construction and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.